The invention relates to a mass-sensitive sensor consisting of an acoustic surface wave (OFW) component on the basis of shear waves.
A way to increase the sensitivity and, consequently, to improve the detection limits of acoustic sensors is to use a special type of waves, the Love-waves.
Love-waves are generally acoustic modes which propagate in a layer structure, which consists of a substrate and a thin wave-conducting layer (homogeneous, or respectively, compact film whose thickness is in the outer of the wavelength) disposed on the substrate.
In order to stimulate Love-waves, the transversal acoustic sound velocity in the deposited layer must be smaller than that in the piezo-electric substrate. Only under this condition, it is possible to transfer a surface skimming bulk wave (SSBW) into a Love-wave. The greater the difference between the two velocities is the more efficient is the conversion of the wave types and, consequently, the greater is the sensitivity gain.
In the case of shear wave components, the surface wave generated on a substrate can be concentrated, by the acoustically softer layer material on the surface (if no periodic mass grids are present) or, respectively, they can be additionally concentrated (if periodic mass grids are present), which results in an increased sensitivity of this wave type to disturbances at the surface and which, in this way, provides for a high mass sensitivity.
Abbreviations which will be used herein:    LW-Component=a component based on Love-waves    SSBW=shear wave (Surface Skimming Bulk Wave)    RW=Rayleigh-waves    OFW/SAW=surface wave